Power transmission apparatus

ABSTRACT

In a power transmission apparatus, an auxiliary clutch plate different in diameter from driving and driven clutch plates located in a clutch housing. When the driving and driven clutch plates are pressed against each other, the auxiliary clutch plate is pressed to enable transmission of a driving force of an engine to a driving wheel. When a pressing force exerted on the driving and driven clutch plates is released, a pressing force exerted on the auxiliary clutch plate is released to cut off transmission of the driving force of the engine to the driving wheel.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a power transmission apparatus that is able to freely transmit a rotational force of an input to an output or cut off the rotational force.

2. Description of the Related Art

Usually, a power transmission apparatus included in a motorcycle freely transmits a driving force of an engine to a transmission and a driving wheel or cuts off the driving force. Such a power transmission apparatus includes an input connected to an engine; an output connected to a transmission and a driving wheel; a clutch member connected to the output; and a pressure member that is able to move close to or away from the clutch member. Bringing the pressure member close to the clutch member presses driving and driven clutch plates against each other so as to transmit power therebetween. Moving the pressure member away from the clutch member releases a pressing force exerted on the driving and driven clutch plates so as to cut off transmission of the power therebetween.

As disclosed, for example, in WO 2013/183588, a power transmission apparatus known in the art includes a centrifugal clutch means including a weight member that moves from a radially inner position of a groove to a radially outer position thereof with centrifugal force produced by rotation of a clutch housing and is thus able to press driving and driven clutch plates against each other. The power transmission apparatus known in the art is able to apply centrifugal force to the weight member in accordance with the rotation of the clutch housing caused by driving of an engine and is thus able to press the driving and driven clutch plates against each other so as to transmit a driving force of the engine to a wheel.

SUMMARY OF THE INVENTION

Unfortunately, the power transmission apparatus known in the art may require increasing the number of driving clutch plates and the number of driven clutch plates in order to achieve a clutch capacity required for a multiple-plate clutch. In this case, if the required clutch capacity is achieved by simply increasing the number of driving clutch plates and the number of driven clutch plates in a stacking direction of the multiple-plate clutch, the size of the apparatus in an axial direction may increase, which may make the apparatus larger than a space in a vehicle where the apparatus is installable. Such a problem may occur not only in apparatuses including weight members but also in general multiple-plate clutch type power transmission apparatuses including driving and driven clutch plates.

Preferred embodiments of the present invention provide power transmission apparatuses that each achieve an increased clutch capacity while avoiding being increased in size in an axial direction.

A power transmission apparatus according to a preferred embodiment of the present invention includes a clutch housing that rotates together with an input that rotates due to a driving force of an engine of a vehicle, the clutch housing including a plurality of driving clutch plates attached thereto, a clutch including a plurality of driven clutch plates attached thereto, the driven clutch plates being alternately arranged with the driving clutch plates attached to the clutch housing, the clutch being connected to an output that is able to rotate a wheel of the vehicle, and a pressure applicator movable between an operating position where the driving and driven clutch plates are pressed against each other so as to enable transmission of the driving force of the engine to the wheel and a non-operating position where a pressing force exerted on the driving and driven clutch plates is released so as to cut off transmission of the driving force of the engine to the wheel. An auxiliary clutch plate different in diameter from the driving and driven clutch plates is included in the clutch housing. When the driving and driven clutch plates are pressed against each other, the auxiliary clutch plate is pressed so as to enable transmission of the driving force of the engine to the wheel. When the pressing force exerted on the driving and driven clutch plates is released, a pressing force exerted on the auxiliary clutch plate is released so as to cut off transmission of the driving force of the engine to the wheel.

The power transmission apparatus may also include a centrifugal clutch including a mass movable from a radially inner position to a radially outer position due to centrifugal force produced by rotation of the clutch housing. When the mass is located at the radially outer position, the centrifugal clutch presses the auxiliary clutch plate concurrently with pressing the driving and driven clutch plates against each other so as to enable transmission of the driving force of the engine to the wheel. When the mass is located at the radially inner position, the centrifugal clutch releases the pressing force exerted on the auxiliary clutch plate concurrently with releasing the pressing force exerted on the driving and driven clutch plates so as to cut off transmission of the driving force of the engine to the wheel.

The centrifugal clutch may includes a holder to hold the mass such that the mass is movable between the radially inner position and the radially outer position, a press to press the driving and driven clutch plates against each other by moving in a stacking direction of the driving and driven clutch plates in response to movement of the mass from the radially inner position to the radially outer position, and a spring to urge the mass from the radially outer position to the radially inner position. When the mass is located at the radially outer position, one of the press and the holder presses the driving and driven clutch plates against each other, and the other one of the press and the holder presses the auxiliary clutch plate.

The auxiliary clutch plate may be positioned such that the auxiliary clutch plate overlaps, in an axial direction of the clutch housing, with a region where the driving and driven clutch plates are stacked.

The auxiliary clutch plate may include a plurality of clutch plates.

According to a preferred embodiment of the present invention, the auxiliary clutch plate different in diameter from the driving and driven clutch plates is provided in the clutch housing. When the driving and driven clutch plates are pressed against each other, the auxiliary clutch plate is pressed so as to enable transmission of the driving force of the engine to the wheel. When the pressing force exerted on the driving and driven clutch plates is released, the pressing force exerted on the auxiliary clutch plate is released so as to cut off transmission of the driving force of the engine to the wheel. Consequently, this preferred embodiment is able to increase a clutch capacity while preventing the apparatus from increasing in size in the axial direction.

According to another preferred embodiment of the present invention, the power transmission apparatus includes the centrifugal clutch configured to, when the mass is located at the radially outer position, press the auxiliary clutch plate concurrently with pressing the driving and driven clutch plates against each other so as to enable transmission of the driving force of the engine to the wheel, and configured to, when the mass is located at the radially inner position, release the pressing force exerted on the auxiliary clutch plate concurrently with releasing the pressing force exerted on the driving and driven clutch plates so as to cut off transmission of the driving force of the engine to the wheel. Consequently, with the centrifugal clutch, this preferred embodiment is able to press the auxiliary clutch plate or release the pressing force exerted thereon, in addition to pressing the driving and driven clutch plates against each other or releasing the pressing force exerted thereon.

According to a further preferred embodiment, when the mass is located at the radially outer position, one of the press and the holder presses the driving and driven clutch plates against each other, and the other one of the press and the holder presses the auxiliary clutch plate. Consequently, with the press and the holder included in the centrifugal clutch, this preferred embodiment is able to press the auxiliary clutch plate or release the pressing force exerted thereon, in addition to pressing the driving and driven clutch plates against each other or releasing the pressing force exerted thereon.

According to yet another preferred embodiment of the present invention, the auxiliary clutch plate is positioned such that the auxiliary clutch plate overlaps, in the axial direction of the clutch housing, with the region where the driving and driven clutch plates are stacked. Consequently, this preferred embodiment is able to reliably prevent the apparatus from increasing in size in the axial direction.

According to an additional preferred embodiment of the present invention, the auxiliary clutch plate includes a plurality of clutch plates. Consequently, this preferred embodiment is able to increase a clutch capacity of the auxiliary clutch plate to any desired level.

The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external view of a power transmission apparatus according to a preferred embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1 .

FIG. 3 is a cross-sectional view taken along the line in FIG. 1 .

FIG. 4 is a perspective view of a clutch housing of the power transmission apparatus.

FIG. 5 is a three-view drawing of a first clutch member of the power transmission apparatus.

FIG. 6 is a perspective view of the first clutch member.

FIG. 7 is a three-view drawing of a second clutch member of the power transmission apparatus.

FIG. 8 is a perspective view of the second clutch member.

FIG. 9 is a three-view drawing of a pressure member of the power transmission apparatus.

FIG. 10 is a perspective view of the pressure member.

FIG. 11 is a longitudinal cross-sectional view of a centrifugal clutch of the power transmission apparatus.

FIG. 12 is a partially cutaway perspective view of the centrifugal clutch.

FIG. 13 is a three-view drawing of a holding member included in the centrifugal clutch.

FIG. 14 is a three-view drawing of a supporting member included in the centrifugal clutch.

FIG. 15 is a three-view drawing of a pressing member included in the centrifugal clutch.

FIG. 16 is a four-view drawing of a weight member included in the centrifugal clutch.

FIG. 17 is a cross-sectional view taken along the line XVII-XVII in FIG. 16 .

FIG. 18 is a plan view illustrating a state of the centrifugal clutch where each weight member is located at a radially inner position.

FIG. 19 is a plan view illustrating a state of the centrifugal clutch where each weight member is located at a radially outer position.

FIG. 20A is a schematic diagram describing how a pressing assist cam of the power transmission apparatus acts.

FIG. 20B is a schematic diagram describing how a back torque limiter cam of the power transmission apparatus acts.

FIG. 21 is a schematic diagram of a vehicle in which the power transmission apparatus is used.

FIG. 22 is a cross-sectional view illustrating a state of the power transmission apparatus where each weight member is located at the radially inner position.

FIG. 23 is a cross-sectional view illustrating a state of the power transmission apparatus where each weight member is located at an intermediate position between the radially inner position and the radially outer position.

FIG. 24 is a cross-sectional view illustrating a state of the power transmission apparatus where each weight member is located at the radially outer position.

FIG. 25 is a cross-sectional view illustrating a state of the power transmission apparatus where each weight member is located at the radially outer position and the pressure member is located at a non-operating position.

FIG. 26 is a longitudinal cross-sectional view of a power transmission apparatus according to another preferred embodiment of the present invention (in which an auxiliary clutch plate includes a plurality of clutch plates).

FIG. 27 is a longitudinal cross-sectional view of a power transmission apparatus according to still another preferred embodiment of the present invention (in which an auxiliary clutch plate includes a plurality of clutch plates and overlaps with a region where driving and driven clutch plates are stacked).

FIG. 28 is a longitudinal cross-sectional view of a power transmission apparatus according to yet another preferred embodiment of the present invention (in which an auxiliary clutch plate includes a plurality of clutch plates, and the apparatus includes no centrifugal clutch).

FIG. 29 is a longitudinal cross-sectional view of a power transmission apparatus according to still yet another preferred embodiment of the present invention (in which an auxiliary clutch plate includes a plurality of clutch plates and overlaps with a region where driving and driven clutch plates are stacked, and the apparatus includes no centrifugal clutch).

FIG. 30 is a longitudinal cross-sectional view of a power transmission apparatus according to another preferred embodiment of the present invention (in which an auxiliary clutch plate includes a plurality of clutch plates, and the apparatus includes no centrifugal clutch but includes a pressing assist cam and a back torque limiter cam).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described below in detail with reference to the drawings. As illustrated in FIG. 21 , a power transmission apparatus K according to the present preferred embodiment is disposed in a vehicle so as to freely transmit a driving force of an engine E to a driving wheel T through a transmission M or cut off the driving force. As illustrated in FIGS. 1 to 17 , the power transmission apparatus K includes a clutch housing 2 provided with an input gear 1 (which is an input) that rotates with the driving force of the engine E of the vehicle, an output shaft 3 (which is an output) connected to the transmission M, a clutch (which includes a first clutch member 4 a and a second clutch member 4 b), a pressure member 5 (which is a pressure applicator), a plurality of driving clutch plates 6, a plurality of driven clutch plates 7, a centrifugal clutch 9 including weight members 10 (which are masses), and an auxiliary clutch plate 17.

The input gear 1 is rotatable around the output shaft 3 upon receiving a driving force (or a rotational force) transmitted from the engine E. The input gear 1 is connected to the clutch housing 2 with a fastener, such as a rivet. The clutch housing 2 is a cylindrical member with an opening defined at its right end in FIGS. 2 and 3 and is connected to the input gear 1. The clutch housing 2 is rotatable together with rotation of the input gear 1 with the driving force of the engine E.

As illustrated in FIG. 4 , the clutch housing 2 is circumferentially provided with a plurality of cut-outs 2 a. The driving clutch plates 6 are fitted to the cut-outs 2A and thus attached to the clutch housing 2. The driving clutch plates 6 are each made of a substantially annular plate material. The driving clutch plates 6 are rotatable together with rotation of the clutch housing 2. The driving clutch plates 6 are slidable in an axial direction (which corresponds to a right-left direction in FIGS. 2 and 3).

The driven clutch plates 7 are attached to the clutch (which includes the first clutch member 4 a and the second clutch member 4 b). The driven clutch plates 7 are arranged alternately with the driving clutch plates 6 attached to the clutch housing 2. The clutch is connected to the output shaft 3 (i.e., the output) that is able to rotate the driving wheel T through the transmission M of the vehicle. The clutch is provided by assembling two members, i.e., the first clutch member 4 a and the second clutch member 4 b, to each other.

The output shaft 3 is inserted through an insertion hole (see FIGS. 5 and 6 ) defined in the center of the first clutch member 4 a. A gear provided on the first clutch member 4 a and a gear provided on the output shaft 3 mesh with each other so as to be connected to each other in the direction of rotation. As illustrated in FIGS. 5 and 6 , the first clutch member 4 a is provided with inclined surfaces 4 aa each defining a pressing assist cam, and inclined surfaces 4 ab each defining a back torque limiter cam. The reference signs “4 ac” in FIGS. 5 and 6 each indicate a boss provided with an insertion hole for a bolt B for connection between the first clutch member 4 a and a securing member 8.

As illustrated in FIGS. 7 and 8 , the second clutch member 4 b is an annular member provided with a flange 4 bb. The driven clutch plates 7 are spline-fitted to a spline-fitting portion 4 ba provided on the outer peripheral surface of the second clutch member 4 b and are thus attached to the second clutch member 4 b. As illustrated in FIGS. 2 and 3 , the pressure member 5 is assembled to the clutch member (which includes the first clutch member 4 a and the second clutch member 4 b) such that the driving and driven clutch plates 6 and 7 are alternately stacked and secured between a flange 5 c of the pressure member 5 and the flange 4 bb of the second clutch member 4 b.

As illustrated in FIGS. 9 and 10 , the pressure member 5 is a disk member provided with the flange 5 c extending along the peripheral edge of the pressure member 5. The pressure member 5 is movable between an operating position where the driving and driven clutch plates 6 and 7 are pressed against each other so as to enable transmission of the driving force of the engine E to the wheel, and a non-operating position where a pressing force exerted on the driving and driven clutch plates 6 and 7 is released so as to cut off transmission of the driving force of the engine E to the wheel.

More specifically, as illustrated in FIGS. 7 and 8 , the spline-fitting portion 4 ba provided on the second clutch member 4 b defines integral projections and depressions along substantially the entire outer peripheral side surface of the second clutch member 4 b. Fitting the driven clutch plates 7 to recessed grooves that define the spline-fitting portion 4 ba restricts movement of the driven clutch plates 7 in the direction of rotation while allowing movement of the driven clutch plates 7 in the axial direction relative to the second clutch member 4 b. The driven clutch plates 7 are rotatable together with the second clutch member 4 b.

The driven clutch plates 7 are stacked alternately with the driving clutch plates 6, making it possible to allow the clutch plates 6 and 7 adjacent to each other to be pressed against each other or release the pressing force exerted thereon. In other words, the clutch plates 6 and 7 are allowed to slide in the axial direction of the second clutch member 4 b. Engaging a clutch by pressing the clutch plates (6 a, 6 b, 7 a, 7 b) against each other enables a rotational force of the clutch housing 2 to be transmitted to the output shaft 3 through the second clutch member 4 b and the first clutch member 4 a. Disengaging the clutch by releasing the pressing force exerted on the clutch plates (6 a, 6 b, 7 a, 7 b) causes the first clutch member 4 a and the second clutch member 4 b to stop following the rotation of the clutch housing 2 such that no rotational force will be transmitted to the output shaft 3.

Accordingly, a state where the driving and driven clutch plates 6 and 7 are pressed against each other enables transmission of a rotational force (i.e., the driving force of the engine E), which is received by the clutch housing 2, to the driving wheel (or the transmission M) through the output shaft 3 (i.e., the output), and a state where the driving and driven clutch plates 6 and 7 are not pressed against each other makes it possible to cut off transmission of the rotational force (or the driving force of the engine E), which is received by the clutch housing 2, to the output shaft 3 (i.e., the output).

As illustrated in FIGS. 9 and 10 , the pressure member 5 includes a plurality of fitting holes 5 d (i.e., three fitting holes 5 d in the present preferred embodiment) circumferentially arranged. A clutch spring S is fitted into each of the fitting holes 5 d. As illustrated in FIG. 2 , one end of each clutch spring S housed in the associated fitting hole 5 d is in abutment with the securing member 8, such that each clutch spring S urges the pressure member 5 in a direction in which the driving and driven clutch plates 6 and 7 are to be pressed against each other. Operating a clutch actuator (not illustrated) makes it possible to press the driving and driven clutch plates 6 and 7 against each other or stop pressing the driving and driven clutch plates 6 and 7 against each other.

In the present preferred embodiment, as illustrated in FIGS. 5, 6, 9, and 10 , the first clutch member 4 a is provided with the inclined surfaces 4 aa and 4 ab, and the pressure member 5 is provided with inclined surfaces 5 a and 5 b respectively facing the inclined surfaces 4 aa and 4 ab. Specifically, each inclined surface 4 aa comes into abutment with the associated inclined surface 5 a so as to provide the pressing assist cam, and each inclined surface 4 ab comes into abutment with the associated inclined surface 5 b so as to provide the back torque limiter cam.

Suppose that the rotation speed of the engine E is increased and the resulting rotational force received by the input gear 1 and the clutch housing 2 is transmittable to the output shaft 3 through the first clutch member 4 a and the second clutch member 4 b (which means that the weight members 10 are each located at a radially outer position). In this case, as illustrated in FIG. 20A, the rotational force is applied to the pressure member 5 in an a-direction, which exerts a force on the pressure member 5 in a c-direction in FIG. 20A under the action of each pressing assist cam. The pressure member 5 is thus moved in a direction in which its flange 5 c comes closer to the flange 4 bb of the second clutch member 4 b (i.e., leftward in FIGS. 2 and 3 ) so as to increase the pressing force exerted on the driving and driven clutch plates 6 and 7.

Suppose that the rotation of the output shaft 3 exceeds the rotation speed of the input gear 1 and the clutch housing 2, resulting in a back torque. In this case, as illustrated in FIG. 20B, a rotational force is applied to the clutch member 4 in a b-direction, which moves the pressure member 5 in a d-direction in FIG. 20B under the action of each back torque limiter cam so as to release the pressing force exerted on the driving and driven clutch plates 6 and 7. This makes it possible to prevent a malfunction in the power transmission apparatus K or a power source (i.e., the engine E) caused by the back torque.

As illustrated in FIGS. 11 to 19 , the centrifugal clutch 9 includes the weight members 10 each movable from a radially inner position (see FIG. 18 ) to the radially outer position (see FIG. 19 ) with centrifugal force produced by rotation of the clutch housing 2. When the weight members 10 are each located at the radially outer position, the centrifugal clutch 9 presses the driving and driven clutch plates 6 and 7 against each other so as to enable transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). When the weight members 10 are each located at the radially inner position, the centrifugal clutch 9 releases the pressing force exerted on the driving and driven clutch plates 6 and 7 so as to cut off transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T).

Specifically, the centrifugal clutch 9 includes the weight members 10, each of which is a substantially symmetrical polygonal member, a holding member 11 (holder) including a supporting member 13 attached thereto, a pressing member 12 (press), first spherical members 14, second spherical members 15, and urging members 16, each of which is a coil spring. The holding member 11 and the pressing member 12 are each circumferentially provided with a plurality of protrusions. Similarly to the driving clutch plates 6, the protrusions are fitted to the cut-outs 2 a of the clutch housing 2 such that the holding member 11 and the pressing member 12 are attached to the clutch housing 2. Accordingly, the holding member 11 and the pressing member 12 are each movable in the axial direction of the clutch housing 2 and in engagement with the clutch housing 2 in the direction of rotation so as to be rotatable together with the clutch housing 2.

As illustrated in FIG. 16 , each weight member 10 is a substantially symmetrical polygonal member including a first surface X and a second surface Y. As illustrated in FIGS. 16 and 17 , each weight member 10 includes through holes 10 a passing through the weight member 10 from the first surface X to the second surface Y, insertion portions 10 b defined in the second surface Y, and a groove 10 c defined in the first surface X. As illustrated in FIGS. 18 and 19 , the weight members 10 are each housed in an associated one of housing portions 11 a of the holding member 11. When no centrifugal force is applied to the weight members 10, the weight members 10 are each held at the radially inner position (see FIG. 18 ). Application of centrifugal force to the weight members 10 moves the weight members 10 outward against the urging force of the urging members 16 such that each weight member 10 reaches the radially outer position (see FIG. 19 ).

The holding member 11 holds the weight members 10 such that each weight member 10 is movable between the radially inner position and the radially outer position. As illustrated in FIG. 13 , the holding member 11 is an annular member. The holding member 11 includes the housing portions 11 a which are arranged in the circumferential direction of the holding member 11 and in which the weight members 10 are to be housed, groove geometries 11 b defined in the housing portions 11 a, and a pushing surface 11 c. Each housing portion 11 a has a recessed shape conforming to the shape and moving range of the associated weight member 10. An inner peripheral wall surface 11 aa of each housing portion 11 a is allowed to abut against first ends of the associated urging members 16.

The supporting member 13 is secured to a surface of the holding member 11, which is provided with the housing portions 11 a. As illustrated in FIG. 14 , the supporting member 13 is provided with radially extending holding portions 13 a. The holding portions 13 a each conform to the groove 10 c of the associated weight member 10, so that the weight members 10 are held by the holding member 11. Specifically, the first surface X of each weight member 10 is centrally provided with the groove 10 c extending in a direction from the radially inner position to the radially outer position. Causing each holding portion 13 a to conform to the associated groove 10 c allows the holding member 11 to hold the weight members 10 such that each weight member 10 is movable in a radial direction (i.e., in a direction from the radially inner position to the radially outer position).

Movement of each weight member 10 from the radially inner position to the radially outer position causes the pressing member 12 to move in a stacking direction of the driving and driven clutch plates 6 and 7 (i.e., rightward in FIGS. 2 and 3 ) such that the driving and driven clutch plates 6 and 7 are pressed against each other. Specifically, as illustrated in FIG. 15 , the pressing member 12 is an annular member. The pressing member 12 includes inclined grooves 12 a arranged in the circumferential direction of the pressing member 12, groove geometries 12 b each defined at a position where the associated inclined groove 12 a is defined, and a pushing surface 12 c.

Each inclined groove 12 a is defined at a position corresponding to the position of the associated weight member 10. Each inclined groove 12 a is inclined upward from its inner portion to its outer portion. Thus, when the clutch housing 2 is stationary, each weight member 10 is held at the radially inner position with the urging force of the associated urging members 16. Rotation of the clutch housing 2 applies centrifugal force to the weight members 10 so as to move the weight members 10 along the inclined grooves 12 a inclined upward. This moves the pressing member 12 away from the holding member 11 (i.e., in the direction in which the driving and driven clutch plates 6 and 7 are to be pressed against each other).

When the holding member 11 and the pressing member 12 are assembled to each other, with the weight members 10 interposed therebetween, each inclined groove 12 a is located at a position corresponding to the position of the associated weight member 10 as illustrated in FIGS. 11 and 12 . With centrifugal force, the weight members 10 each move along the inclined grooves 12 a from the radially inner position to the radially outer position, which moves the pressing member 12 in a direction indicated by the arrow in FIG. 11 (i.e., rightward in FIG. 11 ). The pushing surface 12 c of the pressing member 12 thus pushes the driving and driven clutch plates 6 and 7 such that the driving and driven clutch plates 6 and 7 are pressed against each other. The resulting reaction force moves the holding member 11 in a direction opposite to the direction indicated by the arrow in FIG. 11 (i.e., leftward in FIG. 11 ), so that the pushing surface 11 c of the holding member 11 presses the auxiliary clutch plate 17.

As illustrated in FIGS. 18 and 19 , the weight members 10 according to the present preferred embodiment are each housed in an associated one of the housing portions 11 a arranged in the circumferential direction of the holding member 11 such that the weight members 10 are movable radially. More than one urging member 16 (i.e., two urging members 16 in the present preferred embodiment) is disposed in the circumferential direction between the inner peripheral wall surface 11 aa (see FIG. 13 ) of each of the housing portions 11 a and an associated one of the weight members 10 so as to urge the associated weight member 10 from the radially outer position to the radially inner position. In this preferred embodiment, the inner peripheral wall surface 11 aa of each housing portion 11 a is a flat surface in abutment with the first ends of the associated urging members 16 such that the urging members 16 are stably securable.

The weight members 10 according to the present preferred embodiment are each provided with the tunnel-shaped insertion portions 10 b which are openings defined in a surface of each weight member 10 facing the holding member 11 (i.e., the second surface Y in FIG. 17 ) and into which the urging members 16 are inserted such that the urging members 16 are attachable to the associated weight member 10. The weight members 10, with the urging members 16 inserted into the insertion portions 10 b, are housed in the housing portions 11 a of the holding member 11. The urging members 16 are thus secured such that each urging member 16 is interposed between the inner peripheral wall surface 11 aa of the associated housing portion 11 a and the associated weight member 10. The urging members 16 are disposed such that the first end of each urging member 16 is in abutment with the associated inner peripheral wall surface 11 aa and a second end of each urging member 16 is in abutment with an end wall surface 10 ba of the associated insertion portion 10 b, making it possible to urge each weight member 10 from the radially outer position to the radially inner position.

Each first spherical member 14 is a steel ball attached to the associated weight member 10. As illustrated in FIGS. 16 and 17 , each first spherical member 14 is partially protruded from a first opening 10 aa (i.e., a small-diameter opening in the first surface X) of the through hole 10 a defined in the associated weight member 10 and is in contact with a rolling contact surface of the pressing member 12 so as to be rollable thereon. Each second spherical member 15 is a steel ball attached to the associated weight member 10. As illustrated in FIGS. 16 and 17 , each second spherical member 15 is partially protruded from a second opening 10 ab (i.e., a large-diameter opening in the second surface Y) of the through hole 10 a defined in the associated weight member 10 and is in contact with a rolling contact surface of the holding member 11 so as to be rollable thereon.

As illustrated in FIG. 17 , the through holes 10 a according to the present preferred embodiment are tapered such that the diameter of each through hole 10 a continuously increases from the first opening 10 aa (i.e., the small-diameter opening in the first surface X) to the second opening 10 ab (i.e., the large-diameter opening in the second surface Y). Disconnection of each first spherical member 14 from the associated through hole 10 a is prevented by the outer peripheral edge of one of the first opening 10 aa and the second opening 10 ab that has a smaller diameter (which is, in the present preferred embodiment, the first opening 10 aa in the first surface X). The first and second spherical members 14 and 15 according to the present preferred embodiment are spherical members having different diameters in accordance with the inner diameters of the through holes 10 a. The second spherical members 15 are larger in diameter than the first spherical members 14. The small-diameter first spherical members 14 are each rollable while being in contact with the inner peripheral surface of the associated through hole 10 a, and at the same time, disconnection of the small-diameter first spherical members 14 is prevented by the small-diameter opening edges of the through holes 10 a.

As illustrated in FIGS. 11 and 12 , disconnection of the second spherical members 15 is prevented by the rolling contact surface of the holding member 11. Thus, disconnection of the small-diameter first spherical members 14 is prevented by the small-diameter opening edges of the through holes 10 a, and disconnection of the large-diameter second spherical members 15 is prevented by the rolling contact surface of the holding member 11 while the second spherical members 15 are partially protruded from the large-diameter openings of the through holes 10 a. In the present preferred embodiment, the large-diameter second spherical members 15 are assembled to the weight members 10 such that the second spherical members 15 face the rolling contact surface of the holding member 11. Alternatively, the second spherical members 15 may be assembled to the weight members 10 such that the second spherical members 15 face the rolling contact surface of the pressing member 12. In this case, disconnection of the small-diameter first spherical members 14 is prevented by the small-diameter opening edges of the through holes 10 a, and disconnection of the large-diameter second spherical members 15 is prevented by the rolling contact surface of the pressing member 12 while the second spherical members 15 are partially protruded from the large-diameter openings of the through holes 10 a.

As illustrated in FIG. 13 , the rolling contact surface of the holding member 11 (which is, in the present preferred embodiment, a surface of the holding member 11 on which the second spherical members 15 roll) includes the groove geometries 11 b each extending in the direction of movement of the associated weight member 10 (i.e., a direction connecting the radially inner position to the radially outer position). As illustrated in FIG. 15 , the rolling contact surface of the pressing member 12 (which is, in the present preferred embodiment, a surface of the pressing member 12 on which the first spherical members 14 roll) includes the groove geometries 12 b each extending in the direction of movement of the associated weight member 10 (i.e., a direction connecting the radially inner position to the radially outer position).

As illustrated in FIGS. 16, 18, and 19 , the first and second spherical members 14 and 15 according to the present preferred embodiment are provided such that more than one first spherical member 14 and more than one second spherical member 15 are arranged in the circumferential direction of the holding member 11 (i.e., such that two first spherical members 14 and two second spherical members 15 are arranged in the width direction of each weight member 10 in the present preferred embodiment). In response to movement of the weight members 10, the first spherical members 14 are movable along the groove geometries 12 b while rolling within the through holes 10 a, and the second spherical members 15 are movable along the groove geometries 11 b while rolling within the through holes 10 a.

The auxiliary clutch plate 17 is disposed in the clutch housing 2. The auxiliary clutch plate 17 is an annular member different in diameter from the driving and driven clutch plates 6 and 7 (i.e., smaller in diameter than the driving and driven clutch plates 6 and 7 in the present preferred embodiment). As illustrated in FIGS. 2 and 3 , the output shaft 3 (i.e., the output) is inserted through a central opening 17 a of the auxiliary clutch plate 17 so as to be fitted thereto. The auxiliary clutch plate 17 includes a pushed surface 17 b facing the pushing surface 11 c of the holding member 11.

When the weight members 10 are each located at the radially outer position (i.e., when the driving and driven clutch plates 6 and 7 are pressed against each other), the auxiliary clutch plate 17 is able to transmit the driving force of the engine E to the output shaft 3 upon being pushed by the pushing surface 11 c of the holding member 11 and pressed against the pushing surface 11 c. When the weight members 10 are each located at the radially inner position (i.e., when the pressing force exerted on the driving and driven clutch plates 6 and 7 is released), the auxiliary clutch plate 17 is able to cut off transmission of the driving force of the engine E to the output shaft 3 upon being relieved of a pressing force applied thereto, owing to a decrease in pushing force exerted by the pushing surface 11 c of the holding member 11.

Upon movement of each weight member 10 to the radially outer position, each inclined groove 12 a functions as a cam so as to cause the holding member 11 and the pressing member 12 to move away from each other. Accordingly, the pushing surface 12 c of the pressing member 12 presses the driving and driven clutch plates 6 and 7 against each other, and the pushing surface 11 c of the holding member 11 pushes the pushed surface 17 b of the auxiliary clutch plate 17 such that the pushed surface 17 b is pressed against the pushing surface 11 c so as to transmit the driving force of the engine E to the driving wheel T.

As illustrated in FIGS. 5 and 6 , a portion of a surface of the first clutch member 4 a according to the present preferred embodiment, which faces the pressure member 5, defines an abutment surface 4 ad. As illustrated in FIGS. 9 and 10 , a portion of a surface of the pressure member 5, which faces the first clutch member 4 a, defines an abutment surface 5 e. With the first clutch member 4 a, the second clutch member 4 b, and the pressure member 5 assembled to each other (and with no torque being transmitted from the input gear 1 (i.e., the input) to the output shaft 3 (i.e., the output)), the abutment surface 4 ad and the abutment surface 5 e are in abutment with each other as illustrated in FIGS. 2 and 3 .

With the abutment surface 4 ad and the abutment surface 5 e in abutment with each other as mentioned above, in the course of movement of each weight member 10 of the centrifugal clutch 9 from the radially inner position (see FIG. 22 ) to an intermediate position (see FIG. 23 ) and a resulting increase in torque transmitted from the input gear 1 (i.e., the input) to the output shaft 3 (i.e., the output), the first clutch member 4 a and the pressure member 5 are not allowed to move relative to each other, which restricts operation of the pressing assist cams.

Each weight member 10 of the centrifugal clutch 9 then moves from the intermediate position (see FIG. 23 ) to the radially outer position (see FIG. 24 ), and the driving and driven clutch plates 6 and 7 are pushed by the flange 4 bb of the second clutch member 4 b and thus pressed against each other. When the pushing force of the flange 4 bb is equal to or greater than the urging force of the clutch springs S, the second clutch member 4 b and the pressure member 5 are moved in the axial direction (i.e., rightward in FIGS. 2 and 3 ) relative to the first clutch member 4 a, so that the abutment surface 4 ad of the first clutch member 4 a and the abutment surface 5 e of the pressure member 5 are located away from each other. FIG. 25 illustrates a state in which the weight members 10 are each located at the radially outer position and the pressure member 5 is located at the non-operating position (i.e., a clutch-disengaged state).

With the abutment surface 4 ad and the abutment surface 5 e located away from each other as mentioned above, in the course of movement of each weight member 10 of the centrifugal clutch 9 from the radially inner position to the radially outer position and a resulting increase in torque transmitted from the input gear 1 (i.e., the input) to the output shaft 3 (i.e., the output), the first clutch member 4 a and the pressure member 5 are allowed to move relative to each other, which allows operation of the pressing assist cams.

In the present preferred embodiment, when the driving and driven clutch plates 6 and 7 are pressed against each other, the auxiliary clutch plate 17 is pressed so as to enable transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). When the pressing force exerted on the driving and driven clutch plates 6 and 7 is released, the pressing force exerted on the auxiliary clutch plate 17 is released so as to cut off transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T).

More specifically, the present preferred embodiment involves providing the centrifugal clutch 9 including the weight members 10. When the weight members 10 are each located at the radially outer position, the centrifugal clutch 9 presses the auxiliary clutch plate 17 concurrently with pressing the driving and driven clutch plates 6 and 7 against each other so as to enable transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). When the weight members 10 are each located at the radially inner position, the centrifugal clutch 9 releases the pressing force exerted on the auxiliary clutch plate 17 concurrently with releasing the pressing force exerted on the driving and driven clutch plates 6 and 7 so as to cut off transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T).

In the present preferred embodiment, in particular, when the weight members 10 are each located at the radially outer position, the pressing member 12 and the holding member 11 move away from each other, the pressing member 12 presses the driving and driven clutch plates 6 and 7 against each other, and the holding member 11 presses the auxiliary clutch plate 17. Alternatively, the centrifugal clutch 9 may be disposed such that when the weight members 10 are each located at the radially outer position, the pressing member 12 and the holding member 11 move away from each other, the holding member 11 presses the driving and driven clutch plates 6 and 7 against each other, and the pressing member 12 presses the auxiliary clutch plate 17.

In the present preferred embodiment, the auxiliary clutch plate 17 different in diameter from the driving and driven clutch plates 6 and 7 is disposed in the clutch housing 2. When the driving and driven clutch plates 6 and 7 are pressed against each other, the auxiliary clutch plate 17 is pressed so as to enable transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). When the pressing force exerted on the driving and driven clutch plates 6 and 7 is released, the pressing force exerted on the auxiliary clutch plate 17 is released so as to cut off transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). Accordingly, the present preferred embodiment is able to increase a clutch capacity while preventing the apparatus from increasing in size in the axial direction.

The auxiliary clutch plate 17 according to the present preferred embodiment, in particular, is disposed radially inward of a region where the driving and driven clutch plates 6 and 7 are stacked (which defines a multiple-plate clutch portion) inside the clutch housing 2. Accordingly, the present preferred embodiment is able to increase a clutch capacity while preventing the apparatus from increasing in size not only in the axial direction but also in a direction perpendicular to the axial direction, and to effectively utilize a radially inner space inside the clutch housing 2.

The present preferred embodiment involves providing the centrifugal clutch 9 configured to, when the weight members 10 are each located at the radially outer position, press the auxiliary clutch plate 17 concurrently with pressing the driving and driven clutch plates 6 and 7 against each other so as to enable transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T), and configured to, when the weight members 10 are each located at the radially inner position, release the pressing force exerted on the auxiliary clutch plate 17 concurrently with releasing the pressing force exerted on the driving and driven clutch plates 6 and 7 so as to cut off transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). Accordingly, with the centrifugal clutch 9, the present preferred embodiment is able to press the auxiliary clutch plate 17 or release the pressing force exerted thereon, in addition to pressing the driving and driven clutch plates 6 and 7 against each other or releasing the pressing force exerted thereon.

When the weight members 10 are each located at the radially outer position, one of the pressing member 12 and the holding member 11 presses the driving and driven clutch plates 7 and 12 against each other, and the other one of the pressing member 12 and the holding member 11 presses the auxiliary clutch plate 17. Accordingly, with the pressing member 12 and the holding member 11 included in the centrifugal clutch 9, the present preferred embodiment is able to press the auxiliary clutch plate 17 or release the pressing force exerted thereon, in addition to pressing the driving and driven clutch plates 6 and 7 against each other or releasing the pressing force exerted thereon.

In the power transmission apparatus K according to the present preferred embodiment, the through holes 10 a of the weight members 10 included in the centrifugal clutch 9 each have a tapered shape from the first opening 10 aa to the second opening 10 ab. Disconnection of each first spherical member 14 is prevented by the outer peripheral edge of one of the associated first opening 10 aa and the associated second opening 10 ab that has a smaller diameter. Accordingly, the present preferred embodiment is able to easily and accurately attach the first spherical members 14 to the weight members 10, resulting in a reduction in manufacturing cost.

The first and second spherical members 14 and 15 are spherical members having different diameters in accordance with the inner diameters of the through holes 10 a. The first and second spherical members 14 and 15 are rollable while being in contact with the inner peripheral surfaces of the through holes 10 a. Thus, during movement of the weight members 10, the first and second spherical members 14 and 15 are rollable in a stable manner such that the weight members 10 move smoothly. Disconnection of the second spherical members 15 according to the present preferred embodiment is prevented by the rolling contact surface of the holding member 11 or the pressing member 12. Consequently, assembling the holding member 11 and the pressing member 12 to each other facilitates preventing disconnection of the first and second spherical members 14 and 15.

The rolling contact surface of the holding member 11 or the pressing member 12 includes the groove geometries (11 b, 12 b) extending in the direction of movement of the weight members 10. Accordingly, the present preferred embodiment enables smoother movement of the weight members 10 while reliably preventing disconnection of the second spherical members 15 from the large-diameter openings and disconnection of the first spherical members 14 from the small-diameter openings.

In addition, the weight members 10 according to the present preferred embodiment are each housed in an associated one of the housing portions 11 a arranged in the circumferential direction of the holding member 11 and are thus movable radially. More than one urging member 16 is disposed in the circumferential direction between the inner peripheral wall surface 11 aa of each of the housing portions 11 a and an associated one of the weight members 10 so as to urge the associated weight member 10 from the radially outer position to the radially inner position. Accordingly, the present preferred embodiment is able to accurately urge each weight member 10 from the radially outer position to the radially inner position, enabling stable movement of the weight members 10 in accordance with centrifugal force.

The weight members 10 according to the present preferred embodiment are each provided with the insertion portions 10 b which are openings defined in the surface of each weight member 10 facing the holding member 11 and into which the urging members 16 are inserted such that the urging members 16 are attachable to the associated weight member 10. This facilitates assembling the urging members 16 to the weight members 10. The weight members 10 according to the present preferred embodiment are each provided with the groove 10 c extending in a direction from the radially inner position to the radially outer position. The holding member 11 (or specifically, the supporting member 13 secured to the holding member 11 so as to be integral therewith) is provided with the holding portions 13 a each conforming to the associated groove 10 c and holding the associated weight member 10. Accordingly, the present preferred embodiment enables stable movement of the weight members 10.

The centrifugal clutch 9 according to the present preferred embodiment includes the first spherical members 14 that are partially protruded from the first openings 10 aa of the through holes 10 a defined in the weight members 10 and are in contact with the rolling contact surface (or the groove geometries 11 b) of the holding member 11 so as to be rollable thereon, and the second spherical members 15 that are partially protruded from the second openings 10 ab of the through holes 10 a defined in the weight members 10 and are in contact with the rolling contact surface (or the groove geometries 12 b) of the pressing member 12 so as to be rollable thereon. Accordingly, the present preferred embodiment enables more stable movement of the weight members 10.

The holding member 11 or the pressing member 12, in particular, includes the groove geometries (11 b, 12 b) extending in the direction of movement of the weight members 10. The groove geometries (11 b, 12 b) define the rolling contact surface for the first spherical members 14 or the second spherical members 15 so as to enable smoother movement of the weight members 10. The first and second spherical members 14 and 15 according to the present preferred embodiment are provided such that more than one first spherical member 14 and more than one second spherical member 15 are arranged in the circumferential direction of the holding member 11 (i.e., the width direction of each weight member 10). Accordingly, the present preferred embodiment enables more stable movement of the weight members 10.

Although the present preferred embodiment has been described thus far, the present invention is not limited to this preferred embodiment. As illustrated in FIG. 26 , for example, the auxiliary clutch plate 17 may include a plurality of clutch plates. A multiple-plate clutch provided in this case includes a first auxiliary clutch plate 17 c and a second auxiliary clutch plate 17 d in addition to the auxiliary clutch plate 17 connected to the output shaft 3. Movement of each weight member 10 of the centrifugal clutch 9 to the radially outer position moves the holding member 11. This causes the pushing surface 11 c of the holding member 11 to push the second auxiliary clutch plate 17 d such that the auxiliary clutch plate 17, the first auxiliary clutch plate 17 c, and the second auxiliary clutch plate 17 d are pressed against each other, enabling transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). Providing the auxiliary clutch plate 17 including a plurality of clutch plates in this manner makes it possible to increase a clutch capacity of the auxiliary clutch plate 17 (which is a multiple-plate clutch including the first auxiliary clutch plate 17 c and the second auxiliary clutch plate 17 d) to any desired level.

As illustrated in FIG. 27 , the auxiliary clutch plate 17 may include a plurality of clutch plates (or may be a multiple-plate clutch including first auxiliary clutch plates 17 c and second auxiliary clutch plates 17 d) and may overlap, in an axial direction X of the clutch housing 2, with the region where the driving and driven clutch plates 6 and 7 are stacked. Disposing the auxiliary clutch plate 17 such that the auxiliary clutch plate 17 overlaps, in the axial direction X of the clutch housing 2, with the region where the driving and driven clutch plates 6 and 7 are stacked in this manner makes it possible to reliably prevent the apparatus from increasing in size in the axial direction X.

As illustrated in FIG. 28 , the auxiliary clutch plate 17 may include a plurality of clutch plates, and the apparatus may include an interlocking member 18 instead of the centrifugal clutch. As illustrated in FIG. 29 , the auxiliary clutch plate 17 may include a plurality of clutch plates and overlap with the region where the driving and driven clutch plates 6 and 7 are stacked, and the apparatus may include the interlocking member 18 instead of the centrifugal clutch. As illustrated in FIG. 30 , the auxiliary clutch plate 17 may include a plurality of clutch plates, and the apparatus may include the interlocking member 18 instead of the centrifugal clutch and may also include a pressing assist cam and a back torque limiter cam. When the pressure member 5 is moved such that the driving and driven clutch plates 6 and 7 are pressed against each other, the resulting pressing force moves the interlocking member 18 toward the auxiliary clutch plate 17 such that the interlocking member 18 presses the auxiliary clutch plate 17.

In the present preferred embodiment, disconnection of the second spherical members 15 is prevented by the rolling contact surface (or the groove geometries 11 a) of the holding member 11 (or the pressing member 12). Alternatively, disconnection of the second spherical members 15 may be prevented by any other disconnection preventing structures and methods, such as swaging. In the present preferred embodiment, with the first clutch member 4 a, the second clutch member 4 b, and the pressure member 5 assembled to each other (and with no torque being transmitted from the input gear 1 (i.e., the input) to the output shaft 3 (i.e., the output)), the abutment surface 4 ad and the abutment surface 5 e are in abutment with each other. Alternatively, the first clutch member 4 a may include no abutment surface 4 ad, the pressure member 5 may include no abutment surface 5 e, and the first clutch member 4 a and the pressure member 5 may be spaced away from each other. The power transmission apparatuses according to preferred embodiments of the present invention may find applications as various multiple-plate clutch type power transmission apparatuses for, for example, motorcycles, automobiles, three-wheel or four-wheel buggies, or general purpose machines.

A power transmission apparatus includes an auxiliary clutch plate in a clutch housing. The auxiliary clutch plate is different in diameter from driving and driven clutch plates. When the driving and driven clutch plates are pressed against each other, the auxiliary clutch plate is pressed so as to enable transmission of a driving force of an engine to a wheel. When a pressing force exerted on the driving and driven clutch plates is released, a pressing force exerted on the auxiliary clutch plate is released so as to cut off transmission of the driving force of the engine to the wheel. Such a power transmission apparatus may find applications involving, for example, change(s) in external shape or addition of other function(s).

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

The invention claimed is:
 1. A power transmission apparatus comprising: a clutch housed in a clutch housing that rotates together with an input that rotates due to a driving force of an engine of a vehicle, the clutch housing including a plurality of driving clutch plates attached thereto, the clutch being provided on its outer peripheral surface with a spline-fitting portion to which inner peripheral edges of a plurality of driven clutch plates are fitted, the driven clutch plates being alternately arranged with the driving clutch plates, the clutch being connected to an output that is able to rotate a wheel of the vehicle; and a pressure applicator movable between an operating position where the driving and driven clutch plates are pressed against each other so as to enable transmission of the driving force of the engine to the wheel and a non-operating position where a pressing force exerted on the driving and driven clutch plates is released so as to cut off transmission of the driving force of the engine to the wheel, the pressure applicator being movable relative to the clutch including the spline-fitting portion; wherein an auxiliary clutch plate smaller in diameter than the inner peripheral edges of the driven clutch plates is located on the output; when the driving and driven clutch plates are pressed against each other, the auxiliary clutch plate is pressed to enable transmission of the driving force of the engine to the wheel; and when the pressing force exerted on the driving and driven clutch plates is released, a pressing force exerted on the auxiliary clutch plate is released to cut off transmission of the driving force of the engine to the wheel.
 2. The power transmission apparatus according to claim 1, further comprising a centrifugal clutch including a mass movable from a radially inner position to a radially outer position with centrifugal force produced by rotation of the clutch housing; wherein when the mass is located at the radially outer position, the centrifugal clutch presses the auxiliary clutch plate concurrently with pressing the driving and driven clutch plates against each other so as to enable transmission of the driving force of the engine to the wheel; and when the mass is located at the radially inner position, the centrifugal clutch releases the pressing force exerted on the auxiliary clutch plate concurrently with releasing the pressing force exerted on the driving and driven clutch plates so as to cut off transmission of the driving force of the engine to the wheel.
 3. The power transmission apparatus according to claim 2, wherein at least a portion of the centrifugal clutch is located between the auxiliary clutch plate and the driving and driven clutch plates.
 4. The power transmission apparatus according to claim 2, wherein the centrifugal clutch includes: a holder to hold the mass such that the mass is movable between the radially inner position and the radially outer position; a press to force the driving and driven clutch plates against each other by moving in a stacking direction of the driving and driven clutch plates in response to movement of the mass from the radially inner position to the radially outer position; and a spring to urge the mass from the radially outer position to the radially inner position; and when the mass is located at the radially outer position, one of the press and the holder presses the driving and driven clutch plates against each other, and the other one of the press and the holder presses the auxiliary clutch plate.
 5. The power transmission apparatus according to claim 1, wherein the auxiliary clutch plate is positioned such that the auxiliary clutch plate and a region where the driving and driven clutch plates are stacked are deviated from each other in an axial direction of the clutch housing.
 6. The power transmission apparatus according to claim 1, wherein the auxiliary clutch plate is adjacent to a surface of the clutch located on a first side in an axial direction of the clutch; and the surface of the clutch located on the first side in the axial direction of the clutch is provided with a groove extending in a radial direction of the output shaft.
 7. A power transmission apparatus comprising: a clutch housed in a clutch housing that rotates together with an input that rotates due to a driving force of an engine of a vehicle, the clutch housing including a plurality of driving clutch plates attached thereto, the clutch being provided on its outer peripheral surface with a spline-fitting portion to which inner peripheral edges of a plurality of driven clutch plates are fitted, the driven clutch plates being alternately arranged with the driving clutch plates, the clutch being connected to an output that is able to rotate a wheel of the vehicle; and a pressure applicator movable between an operating position where the driving and driven clutch plates are pressed against each other so as to enable transmission of the driving force of the engine to the wheel and a non-operating position where a pressing force exerted on the driving and driven clutch plates is released so as to cut off transmission of the driving force of the engine to the wheel; wherein an auxiliary clutch plate smaller in diameter than the inner peripheral edges of the driven clutch plates is located on the output; when the driving and driven clutch plates are pressed against each other, the auxiliary clutch plate is pressed to enable transmission of the driving force of the engine to the wheel; when the pressing force exerted on the driving and driven clutch plates is released, a pressing force exerted on the auxiliary clutch plate is released to cut off transmission of the driving force of the engine to the wheel; and the auxiliary clutch plate includes a plurality of clutch plates each smaller in diameter than the inner peripheral edges of the driven clutch plates.
 8. A power transmission apparatus comprising: a clutch housed in a clutch housing that rotates together with an input that rotates due to a driving force of an engine of a vehicle, the clutch housing including a plurality of driving clutch plates attached thereto, the clutch including a plurality of driven clutch plates attached thereto, the driven clutch plates being alternately arranged with the driving clutch plates, the clutch being connected to an output that is able to rotate a wheel of the vehicle; a pressure applicator movable between an operating position where the driving and driven clutch plates are pressed against each other so as to enable transmission of the driving force of the engine to the wheel and a non-operating position where a pressing force exerted on the driving and driven clutch plates is released so as to cut off transmission of the driving force of the engine to the wheel; an auxiliary clutch plate on the output shaft; and a centrifugal clutch including a mass movable from a radially inner position to a radially outer position due to centrifugal force produced by rotation of the clutch housing, the centrifugal clutch being provided between the auxiliary clutch plate and the driving and driven clutch plates; wherein when the mass is located at the radially outer position, the centrifugal clutch presses the auxiliary clutch plate concurrently with pressing the driving and driven clutch plates against each other so as to enable transmission of the driving force of the engine to the wheel; and when the mass is located at the radially inner position, the centrifugal clutch releases a pressing force exerted on the auxiliary clutch plate concurrently with releasing the pressing force exerted on the driving and driven clutch plates so as to cut off transmission of the driving force of the engine to the wheel.
 9. The power transmission apparatus according to claim 8, wherein the clutch is provided on its outer peripheral surface with a spline-fitting portion to which inner peripheral edges of the driven clutch plates are fitted; and the auxiliary clutch plate is smaller in diameter than the inner peripheral edges of the driven clutch plates.
 10. The power transmission apparatus according to claim 8, wherein the auxiliary clutch plate overlaps with at least a portion of the centrifugal clutch as viewed in a radial direction of the output.
 11. The power transmission apparatus according to claim 8, wherein the driving clutch plates and the driven clutch plates are located on a first side relative to the auxiliary clutch plate with respect to an axial direction of the output; and a portion of the centrifugal clutch is located on a second side opposite to the first side relative to a surface of the auxiliary clutch plate on the first side. 